Some plants, such as pine and grass, are wind-pollinated, so their reproductive strategy is to produce large amounts of pollen in hopes that some makes it to the female. Many other plants depend on animals to spread their pollen. In that case, the animal involved is called a pollinator. Not all animals can pollinate all plants, but certain types of animals such as birds, butterflies, moths, bees, beetles, wasps, bats, and flies, typically pollinate certain types of plants. This is a mutualistic relationship where both the plant and the pollinator(s) benefit each other. Some plants are very specific with respect to what animal is able to serve as a pollinator, and have special modifications (special shape, etc.) to attract that pollinator or exclude other would-be pollinators. Others plants are more general and are more attractive to a wider variety of pollinators, but the risk here is that the pollen may not get to the “right” species if the pollinator visits a different type of flower next.
There are special cases where a plant species and a species of pollinator are totally dependent on each other. These cases are examples of coevolution, the joint evolution of a plant and its animal pollinator. In coevolution, each of the species involved serves as a source of natural selective pressure on the other. A more formal definition for coevolution is “the mutual evolutionary influence between two species.” One example of this type of coevolution would be the yucca plant and the yucca moth. The female moth lays her eggs in the flowers, simultaneously pollinating the plant, and the caterpillars develop within the seeds in the ovary of the plant. For the plant, the loss of a few seeds to caterpillars is a price worth paying to insure pollination. The yucca moth is the only animal that is the right size and shape to pollinate yucca flowers.
In order to make use of animal pollinators, plants must:
- supply some reward, frequently food, for the pollinator,
- advertise the presence of this to attract visitors, and
- have a way of putting pollen on the pollinator so it is transferred to the next plant/flower.
The “reward” is not always food (nectar). There is a tropical orchid with a flower that looks and smells like the female of a certain species of wasp. Males of this species emerge one week before the females, but these orchids are already blooming. The male wasps smell the orchids, “think” they’ve found a female, and try to copulate. The texture of the flowers is such that they “feel” like a female wasp, but the poor males just can’t get it to work, leave to find a more cooperative mate, and end up transferring pollen instead.
The adaptations exhibited by any given flower depend on the type of pollinator the flower is designed to attract. Various pollinators have differing adaptations and means of gathering pollen and/or whatever nectar, etc. a flower has to offer.
Bees don’t see red, but do see blue, yellow, and ultraviolet. Thus, bee-pollinated flowers are mostly yellow (some blue) with ultraviolet nectar guides or “landing patterns.” The flowers typically have a delicate, sweet scent, which the bees can smell. Usually the nectar is at the end of some type of small, narrow floral tube which is the right length to fit the tongue of the particular species bee that pollinates that plant. Bee-pollinated flowers typically have a sturdy, irregular shape with some type of specifically-designed landing platform. An example of this is snapdragons, where only a bee of just the right size and weight is able to trigger the flower to open, while all others (which are too small or too heavy) are excluded. Typically, pollen sticks to the “fur” of a bee or else the bees collect the pollen in specially-modified areas on their legs.
Butterflies are diurnal and have good vision but a weak sense of smell. They can see red. Butterfly-pollinated flowers are brightly-colored (even red) but odorless. These flowers are often in clusters and/or are designed to provide a landing platform. Butterflies typically walk around on a flower cluster, probing the blossoms with their tongues. Examples of butterfly-pollinated flowers would be many members of the plant family Compositae, where many small flowers are arranged into a flat-topped head, and other plants, such as the milkweeds, where the flowers occur in large clusters. The individual flowers are typically tubular with a tube of suitable length for butterflies.
Most moths are nocturnal and have a good sense of smell. Moth-pollinated flowers typically are white or pale colors so they will be at least somewhat visible on a moonlit night. Often, moth-pollinated flowers may only be open at night. They typically use a strong, sweet perfume to advertise their presence in the darkness, and typically this odor is only exuded at night (evolutionarily, it doesn’t make sense to waste energy producing attractant in the daytime when it is useless). Moths are hover-feeders, so these flowers have deep tubes to precisely match the length of a specific moth’s tongue. One famous story relates that Charles Darwin found an extrordinarily long, tubular flower in South America and predicted that some day, someone would find a moth with a tongue of matching length. After much searching, around a hundred years later, indeed, this moth was found. More recently, a flower with an even-longer tube was found on Madagascar, and Dr. Gene Kritsky out at Mt. St. Joe has been interested in trying to find the “missing” moth that goes with it. In moth-pollinated flowers, the petals are flat or bent back so the moth can get in, and hover close to the flower.
Birds, especially hummingbirds, have good eyes and seem to be especially attracted to red. However, birds have a poor sense of smell (yes, it is OK to carefully put fallen babies back into the nest — the parents cannot smell your scent). Bird-pollinated flowers are brightly-colored, especially red but lack odor. Their petals are recurved to be out of the way. Hummingbirds are hover-feeders, so the flowers are designed to dust the bird’s head/back with pollen as the bird probes the flower for nectar. Flowers such as Columbine, red Salvia, and Fuchsia are favorite nectar sources for hummingbirds.
Bats are nocturnal with a good sense of smell. While many bats depend on echolocation rather than sight to navigate, those species which serve as pollinators do have good vision. Also, bats which pollinate flowers have long, bristly tongues to lap up nectar and pollen. Since these flowers are open at night, they are white or light-colored so they’ll be visible in moonlight. Bat-pollinated flowers have a musty smell like the smell of bats. These flowers are large and sturdy to withstand insertion of the bat’s head as it licks nectar and pollen.
Flies are attracted to rotting meat. Thus, fly-pollinated flowers may be nondescript or brownish red in color, and typically have a strong, “bad,” rotten sort of smell.
